The present invention relates to a pulse transformer for use in switching power supplies which transforms a pulsed rectified voltage derived from a commercial power supply system.
FIG. 1 of the accompanying drawings illustrates one example of a switching power supply circuit. A voltage from a commercial power supply is applied through line filters C1, C2, L1, L2, C3 to a full-wave rectifier D1. A rectified voltage from the full-wave rectifier D1 is smoothed by an electrolytic capacitor C4 and then converted by a switching circuit SW1 into high-frequency pulses which are transformed by a transformer T1. A transformed output from the secondary winding of the transformer T1 is rectified by a diode D2 and the rectified voltage is smoothed by a choke L3 and an electrolytic capacitor C5 into a desired DC voltage. Designated at D3 is a flywheel diode for the choke L3, and C6, C7 are line filters. The illustrated circuit arrangement allows the power supply to be small in size and lightweight. However, since the commercial power supply voltage of 100 V or the like is rectified and switched on and off, there are generated high-frequency pulses of one through several hundred volts across the primary winding of the transformer T1 at all times. Accordingly, common-mode noise sources are liable to occur in the primary and secondary windings of the transformer. As shown in FIG. 2, these noise sources are coupled in a voltage-dividing mode by a capacitance between the primary and secondary windings of the transformer T1, and the coupled noise sources serve as a common noise source e.sub.N with respect to ground. The noise is voltage-divided by the input filters L1, L2, C1, C2 and the output filters C6, C7 into primary and secondary common-mode noises which are delivered to the power supply line and the load, respectively. Prevention of these noises from being delivered out requires an increase in the capacitance of the filter capacitors C1, C2, C5, C7 connected to frame ground F.G. As a result, an increased leakage current flows from the power supply through the capacitors C1, C2, with the result that the prior pulse transformer cannot be used particularly in ME (medical electronics) devices as it lacks a required degree of safety.